\begin{table}[htbp]\centering
\def\sym#1{\ifmmode^{#1}\else\(^{#1}\)\fi}
\caption{Marginal effects of covariates, no controls}
\begin{tabular}{l*{3}{c}}
\hline\hline
            &          US         &          UK         &     Germany         \\
            &                     &                     &                     \\
\hline
Linear time trend&    -0.00088\sym{***}&    -0.00279         &     0.00114\sym{***}\\
            &   (0.00016)         &   (0.00151)         &   (0.00014)         \\
[1em]
Unemployment rate*100&     0.01103\sym{***}&     0.01464\sym{***}&     0.01880\sym{***}\\
            &   (0.00114)         &   (0.00149)         &   (0.00055)         \\
\hline
N           &       20273         &      192029         &      303248         \\
\hline\hline
\multicolumn{4}{l}{\footnotesize Standard errors in parentheses. * p < 0.05, ** p < 0.01, *** p < 0.001}\\
\end{tabular}
\end{table}
